An organization that wants to provide employees at one location with telephones has, in general, two options. As a first option, the organization can acquire one telephone and one telephone line per employee from the telephone company. As a second option, the organization can acquire one telephone per employee, a small number of telephone lines from the telephone company, and a piece of equipment that enables the employee's telephones to share the small number of telephone lines. In general, the second option is substantially less expensive than the first option. The piece of equipment that enables the employees' telephones to share the small number of telephone lines is called a “private-branch exchange” or “PBX.”
FIG. 1 depicts a schematic diagram of telecommunications system 100 in the prior art, which reflects the second option. Telecommunications system 100 comprises affiliated off-premises telecommunications terminals 101-1 through 101-M, wherein M is a positive integer; unaffiliated off-premises telecommunications terminal 102; affiliated on-premises telecommunications terminals 103-1 through 103-P, wherein P is a positive integer; private-branch exchange telephone system 104; and telecommunications network 105, interconnected as shown.
The terms “affiliated” and “unaffiliated,” as they apply to the off-premises terminals, refer to whether an off-premises terminal is affiliated with an on-premises terminal (i.e., a terminal served by private-branch exchange 104). The affiliation of one terminal with another terminal concerns, among other things, whether an incoming call intended for an enterprise user at exchange 104 is delivered to both an on-premises terminal and an off-premises terminal. The relationship of an on-premises terminal such as terminal 103-1 with an affiliated off-premises terminal such as terminal 101-1 is described later and with respect to FIG. 2.
Private-branch exchange 104 is capable of switching incoming calls from telecommunications network 105 (e.g., the Public Switched Telephone Network, etc.) via one or more transmission lines to any of on-premises terminals 103-1 through 103-P. Private-branch exchange 104 is also capable of handling outgoing calls from any of on-premises terminals 103-1 through 103-P to telecommunications network 105 via one or more transmission lines that connect private-branch exchange 104 to telecommunications network 105.
Private-branch exchange 104 is capable of also extending an incoming call to a telephone number in telecommunications network 105, in addition to switching the incoming call to on-premises terminal 103-p, wherein p is an integer between 1 and P, inclusive. The telephone number that is extended-to in telecommunications network 105 corresponds to an affiliated terminal 101-m, wherein m is an integer between 1 and M. Private-branch exchange 104 might also be directly connected to one or more additional PBXs, or other types of switches, via voice trunk group 106, which comprises one or more voice trunks.
In addition, private-branch exchange 104 is capable of providing telecommunications features that enable the forwarding of calls, the transferring of calls, conferencing, etc. Typically each user of an on-premises terminal 103-p can create a customized profile that is stored at private-branch exchange 104 and indicates to private-branch exchange 104 how to present information to and respond to signals from on-premises terminal 103-p and its affiliated terminal 101-m, wherein m is equal to p. A customized profile might include information such as defaults for various settings and options, what language to use for voice prompts, what menu items to present, how to associate number keys to menu items, and so forth.
FIG. 1 also depicts the address spaces that are relevant to telecommunications network 100 in the prior art. The term “address space” refers to an addressable region of telephone service. Address space 111 represents the addressable region served by telecommunications network 105. Address space 112 represents the addressable region served by private-branch exchange 104.
Private-branch exchange 104 acts as a “bridge” between address space 111 and address space 112. When a calling party places a call to someone served by private-branch exchange 104, the calling party uses a dialing sequence that includes a telephone number that belongs to telecommunications network 105 and residing in address space 111. As part of the dialing sequence, the calling party also uses an extension number that allows access to one of the on-premises telecommunications terminals that reside within address space 112.
Thus an on-premises telephone number is one that exists within the address space of the private-branch exchange, and an off-premises telephone number is one that exists within the address space of the Public Switched Telephone Network, or other type of public telecommunications network.
Some private-branch exchanges, such as exchange 104, enable the user of an on-premises terminal to associate an off-premises terminal's telephone number (e.g., the user's cell phone number, etc.) with the on-premises terminal's telephone number for features such as automatic call forwarding. In such instances the off-premises terminal's telephone number is said to be mapped to the on-premises terminal's telephone number. Typically a private-branch exchange that provides such a mapping feature stores the station mappings in a table, such as the one shown in FIG. 2 in which each depicted row corresponds to a different station mapping.
Each station mapping initialized and maintained at private-branch exchange 104 comprises a different “description” of a logical association between i) a telephone extension of an on-premises terminal (i.e., terminal 103-p) within exchange 104's address space and ii) a telephone number of an off-premises terminal (i.e., terminal 101-m) within the address space of the Public-Switched Telephone Network.
For each incoming call, intended for a user of an on-premises terminal, exchange 104 uses the station mapping that corresponds to the particular user being called to determine which off-premises terminal is to be extended the incoming call, in addition to delivering the call to the user's on-premises terminal. Aside from determining how to deliver an incoming call, exchange 104 can also use a station mapping to identify an incoming call. Suppose that a first employee is being called by a second employee who is using his cell phone. The station mapping enables the call from the second employee to be identified to the first employee by the station extension of the second employee's deskset terminal, rather than by the second employee's cell phone number, which is information that might be meaningless to the first employee.